Modern microelectronics has allowed for considerable progress to be made in medical technology in the last thirty years. Due to the increasing miniaturization of electronic components, it has been possible to develop a number of implantable medical devices in recent years for perceiving electrical signals of the human body and providing electrical signals, e.g. in the form of energy shocks or pulses, thereby exerting an influence on biological processes within the human body. Such devices include e.g. implantable cardiac defibrillators or implantable devices for stimulating nerves or muscles, including so-called neurostimulators, which are intended to relieve chronic pain by overstimulating a certain neuromere. Other such devices are stimulators for restoring a muscle tone if endogenic nerve pathways have failed, e.g. in the case of paraplegia. These implantable devices also include so-called defibrillators, which can reestablish a regular cardiac rhythm in the case of ventricular fibrillation by providing an electric shock to the cardiac tissue. These implantable defibrillators save lives in the case of certain asrhythmias.
Defibrillation has been known for more than thirty years. In the early years, it served in particular to help a fibrillating heart back to its regular beating rhythm during open heart surgery; cf. J. B. Rosenbaum in Surgery, May 1955, pp. 712 and 713.
In the early 1980's, a defibrillator was implanted for the first time. Experiences with this implanted device can be found in the survey by L. Watkins et al.: Automatic defibrillation in man, in J. Thorac, Cardiovasc. Surg. 82, 1981, pp. 492 to 500. The implantation method, which has not changed essentially up to now, consisted in applying two electrodes to the myocardium after opening the chest cavity. In addition to this method, one already introduced in early years was that of applying to the heart a usually large-surface electrode, a so-called patch, and placing an opposite electrode transvenously in the vena cava or in the atrium or right ventricle. The surgical method used for this purpose world-wide is also described in the article by L. Watkins et. al. After opening the thorax one exposes the pericardium and the heart, so that one can sew on the patch electrode required for defibrillation.
Since the patients in need of an implantable defibrillator generally have a very poor pumping function of the heart, one must not underestimate the complications of such a great surgical operation. Along with the risks of general anesthesia, problems occurred in past years in particular with respect to the placing of the electrodes and with respect to inadequate energy release or stimulus threshold conditions which even made it necessary to explant the entire defibrillator. There were also wound-healing impairments, pocket infections and large hematomas in the pericardium as a result of the operation. Post and perioperative fatalities were described, and repeatedly an additional further deterioration of the cardiac pumping function due to the sewn on patch electrode. A survey of such problems is found in the article by D. Echt et al., Clinical experience, complications and survival in 70 patients with the automatic implantable cardioverter/defibrillator, in Circulation, vol. 71, no. 2, 1985, pp. 289 to 296.
The patent literature describes, in addition to the above-mentioned patch or pad electrodes, a number of other electrode constructions and arrangements as well as methods of defibrillation.
U.S. Pat. No. 3,614,955 together with reissue patents nos. 27,652 and 27,757 describe an electrode which is introduced into the right ventricle of the heart and serves there not only to give shocks but also to monitor the cardiac activity. An electrode for defibrillation that is also introduced into the right ventricle and disposed on a catheter is known from U.S. Pat. No. 3,942,536. However, these two methods have not gained acceptance in practice due to insufficient functioning.
U.S. Pat. No. 3,857,398 discloses the combination of a defibrillator with a demand pacemaker. The defibrillator is triggered by cardiogenic electrical signals which are perceived with an electrode. The triggering of a defibrillator on the basis of perception of the electrical and mechanical activity of the heart is described in U.S. Pat. No. 4,291,699. U.S. Pat. No. 4,641,656 describes a pad electrode provided on the myocardium in conjunction with a large-surface opposite electrode in the right ventricle, for distributing the current conduction evenly over different muscle areas of the heart. According to U.S. Pat. No. 4,548,203, two pairs of spatially remote electrodes are disposed on the myocardium, whereby ventricular fibrillation and tachyrhythmia can be controlled by two pulses at different times. An endocardial electrode having an electrical connection between different conductor structures with low resistance is described in U.S. Pat. No. 4,481,953. A transvenous electrode arrangement is also known from U.S. Pat. No. 4,355,646, in which an endocardial electrode with two electrode points in the ventricle and two electrode points in the atrium is connected by a triaxial lead with low electrical resistance to an implantable cardioverter or defibrillator, for optimally measuring the changes in impedance of the ventricle and allowing for cardioversion to be performed via the same electrode. U.S. Pat. No. 4,355,642 discloses a disk-shaped electrode arrangement suitable both for perceiving the cardiogenic beat and for defibrillation. According to U.S. Pat. No. 3,738,370, a bipolar coaxial catheter is placed in the atrium, the electrode points thereof being used to perform the defibrillation. In U.S. Pat. No. 4,708,145, a pad electrode is placed on the myocardium while an opposite electrode is disposed within the heart. By sequential stimulation one can therefore eliminate ventricular fibrillation. In U.S. Pat. No. 4,787,389, electrode points are also provided outside on the myocardium as well as in the atrium and in the ventricle. There is a possibility here of performing antitachycardiac stimulation and defibrillation of the heart jointly, coded pulses being provided by the defibrillator to protect the antitachycardiac pacemaker from being damaged by the defibrillation pulse. U.S. Pat. No. 4,774,952 describes a multiple electrode arrangement for improved concentration of the current in the muscular areas of the heart during defibrillation. A similar electrode arrangement having a plurality of electrodes attached to the myocardium is also known from Soviet author's certificate no. 1,263,260. By evenly disposing a plurality of heteropolar electrodes about the heart, one reduces the harmful effects of the current on the myocardial cells. British patent application no. 2,182,566 shows an elastic disk electrode which is applied to the myocardium and, thanks to its elasticity, can better follow the movements of the heart. U.S. Pat. Nos. 4,270,549 and 4,291,707 describe a pad electrode which can be attached to the myocardium. A fine titanium wire structure serves as an electrode pole, whereby the mean current density can be increased by applying lateral insulators. These patents also mention a method by which the patch electrode can be applied without opening the upper chest cavity. This is done with a spatula-like instrument which is introduced into the chest cavity through a cut. However, the dimensions of the electrode presented here make it necessary for this introducing instrument to be of considerable size, having a width of four to six centimeters and a thickness of one to three centimeters. It is understood that this kind of opertion also needs general asesthesia and cannot be performed simply under local anesthesia.
Other patent art representative of the state of the defibrillation art and implantable cardiac electrodes therefor include U.S. Pat. 4,765,341, M. M. Mower, et al. and 4,512,351 P. J. Pohndorf, and European patent application 0,317,490 T. J. Fogarty published May 24, 1989 based upon a U.S. priority date of Nov. 13, 1987.
None of this art has resolved the problems encountered in interfacing electrodes with body tissue for low resistance conductivity, large surface area, low polarization and little intrinsic stiffness with interchange of electrical signals over long life with the capacity for good communication and high current levels. Additionally it still remains complicated to introduce and manipulate prior art electrodes and associated electrical therapeutic instruments.
In particular, the methods and apparatus described in the aforesaid literature and patents have not been able to solve essential aspects and problems of defibrillation of the heart. The basic problem relates to the energy transmission from an electrical device to the excitable human tissue via the electrode arrangement. Excitable tissue is understood to refer to those cells whose membrane field strength can be affected by the application of an electrical current in such a way as to result in a depolarization of the cell.
In the case of a nerve this results in a transmitted pulse, while in the case of a muscle cell a transmitted contraction results. The basic problems of energy transmission between an implantable device and the excitable tissue are accordingly found, as described at the outset, not only in defibrillators but also in neurostimulators, muscle stimulators, cochlear implants, and the like.
On the other hand for implantable cardiac pacemakers the problem of transmitting the energy from the pacemaker electrode to the heart has been intensely investigated in the past. A survey of these problems can be found in the article by A. Ripart and J. Muciga: Electrode heart interface: definition of the ideal electrode, in PACE, vol. 6, Mar. 1983, pp.410 to 421. The energy transmission was acceptably solved by materials with low polarization and an electrode head at the tip of the pacemaker electrode having an internal surface enlargement. The surface area required for the energy exchange between the myocardial tissue and the electrode is 10 square millimeters on the average. This surface suffices for transmitting the pacemaker pulses with voltages between 2.5 and 5 volts to the heart and stimulating the latter to beat. The cardiogenic action can also be perceived via this electrode surface. Materials for such electrodes are platinum iridium, pyrolyzed carbon and similar solids. However, the conditions described there for cardiac pacing are completely different from those requirments to be met by electrode systems for defibrillation of the heart, since with defibrillation a manyfold surface (10,000 mm.sup.2) is mandatory in order to apply voltages of several hundred volts to the heart.
One electrode example is set forth in German Democratic Republic patent No. 263,239 Oct. 30, 1987 wherein a heart pacing lead comprises a bundle of anisotropic carbon fibers, which are effective for transmission of the pulse along the longitudinal axis toward the heart tissue. These electrodes could never support the large electrically active surface area needed in defibrillators because of the very small point contact surface area with heart tissue and would have a tendency to erode in time biologically in the electrode to body interface. Furthermore, this point contact characteristic cannot reliably support electrical signal communication for neurostimulation or muscle stimulation for instance because of a relatively undefined region of communication in body tissue disposed over larger surface areas. There must therefore be in essence a joining of each body nerve junction with a point contact, which makes potential use of such electrodes prohibitive.
Unsolved problems arise in the case of defibrillation of the heart. Here, voltages between 500 and 2000 volts are required, depending on the energy release of the implanted defibrillator, to rectify the heart, that is in a state of chaotic excitation, and restore a regular beating activity by providing shocks and defibrillation. In order to couple such energies into the heart, the defibrillation electrodes must have a surface between 50 and 100 square centimeters to avoid local burns. Transvenously applied electrodes also have a surface between 4 and 20 square centimeters to ensure an even energy flow through the heart. The large surface for the energy exchange, however, is only one of many requirements with respect to an optimal energy transmission between an implantable defibrillator and the heart. Other requirements are low energy consumption due to nonpolarizing materials, and an even larger surface that can expand further, if possible, thereby contacting many structures of the heart. The electrode providing the energy exchange with the heart should furthermore be very flexible and also allow for myocardial contact upon movement of the heart. Such high flexibility also prevents any further restriction of the myocardial function, which is usually already restricted in these patients, whether with respect to the systolic pumping function or with respect to the diastolic relaxation. High flexibility would furthermore restrict a mechanically induced foreign-body reaction. Such improvements with respect to the energy transmission would also allow for more economical utilization of the available energy, thereby making it possible to use smaller implantable devices with a longer life without increasing the battery capacity. The electrode should furthermore be easy to apply in order to minimize the risk of the operation. If possible, the risks of a thoracotomy should be avoided, to put an end to the wound-healing impairments, hemorrhages in the pericardium and other infections occurring in the past. It is desirable to be able to apply an electrode to patients who have already been operated on several times, even if these patients have developed corresponding adhesions due to a previous thoracotomy. An easier application would also result in lower hospital costs and could also be performed in hospitals which do not have their own heart surgery departments. It is also desirable to make the application readily repeatable if, for example, a change in the course of the disease due to new infarctions alters the demands to be made on the electrode arrangement, necessitating e.g. a higher energy exchange.